Normal haemeostasis is the result of a complex balance between the processes of clot initiation, formation and clot dissolution. The complex interactions between blood cells, specific plasma proteins and the vascular surface, maintain the fluidity of blood unless injury and blood loss occurs.
Many significant disease states are related to abnormal haemeostasis. For example, local thrombus formation due to the rupture of atherosclerotic plaque is a major cause of acute myocardial infarction and unstable angina. Treatment of an occlusive coronary thrombus by either thrombolytic therapy or percutaneous angioplasty may be accompanied by acute thrombolytic reclosure of the affected vessel. Furthermore, a high percentage of patients undergoing surgery, particularly in the lower extremities, suffer thrombus formation in the venous vascular system which results in reduced blood flow to the affected area. Each year in the United States, thromboprophylaxis affects approximately 3.3 million patients and deep vein thrombosis occurs in approximately 600,000 patients. Stroke occurs in approximately 5 million patients each year which have episodic atrial fibrillation. Venous thromboembolism, especially in cancer patients, is another manifestation of thrombus disorder.
There continues to be a need for safe and effective therapeutic anticoagulants to limit or prevent thrombus formation.
Blood coagulation is vital for the containment of bodily fluids upon tissue injury and is an important component of host defense mechanisms. Coagulation or clotting involves the sequential activation of multiple zymogens in a process leading to thrombin generation and the conversion of fibrinogen to an impermeable cross-linked fibrin clot. Thrombin production is the result of a blood coagulation cascade which has been intensively studied and increasingly characterized. See for example, Lawson, J. H., et al. (1994) J. Biol. Chem. 269:23357. The coagulation reactions of this cascade involve initiation, amplification and propagation phases. Additionally, the cascade has been divided into extrinsic and intrinsic pathways. The intrinsic coagulation cascade pathway involves factors XII, XI, and IX and leads to the formation of a complex of factor Ixa with its cofactor, factor VIIIa. This complex converts factor X to Xa. Factor Xa is an enzyme which forms a complex with its cofactor, factor Va, and rapidly converts prothrombin to thrombin. Thrombin converts fibrinogen to fibrin monomers which polymerize to form a clot. The extrinsic pathway involves factor VIIa and tissue factor, which form a complex (TF/factor VIIa), and convert factor X to Xa. As in the intrinsic pathway, factor Xa converts prothrombin to thrombin.
Thrombin (factor IIa), as noted above, occupies a central position in the coagulation cascade by converting fibrinogen to fibrin. Consequently, substantial synthetic efforts have been directed to the development of thrombin inhibitors. See, for example, U.S. Pat. Nos. 5,656,600; 5,656,645; 5,670,479; 5,646,165; 5,658,939; 5,658,930 and WO 97/30073. Additional compounds which have been prepared as synthetic thrombin inhibitors are N-arylsulfinated phenylalanine amides.
Approved anticoagulant therapeutics include orally-administered Warfarin (COUMADIN®) and the subcutaneous injectable LMWH (Low Molecular Weight Heparins). Ximelagatran (EXANTA®) is under development (AstraZeneca) as an oral direct thrombin inhibitor for the prevention and treatment of venous thromboembolism (VTE) and for prevention of stroke in patients with atrial fibrillation. Known inhibitors of factor Xa include bisamidine compounds (Katakura, S. (1993) Biochem. Biophys. Res. Commun., 197:965) and compounds based on the structure of arginine (WO 93/15756; WO 94/13693). Phenyl and naphthylsulfonamides have also been shown to be factor Xa inhibitors (WO 96/10022; WO 96/16940; WO 96/40679).
TF/factor VIIa is a serine protease complex that participates in blood coagulation by activating factor X and/or factor IX. Factor VIIa is produced from its precursor, factor VII, which is synthesized in the liver and secreted into the blood where it circulates as a single chain glycopeptide. The cDNA sequence for factor VII has been characterized (Hagen et al. (1986) Proc. Natl. Acad. Sci. U.S.A., 83:2412–2416).
A variety of natural and synthetic inhibitors of TF/factor VIIa are known and have varying potency and selectivity. Tissue factor pathway inhibitor (TFPI; Broze, 1995, Thromb. Haemostas., 74:90) and nematode anticoagulant peptide c2 (NAPc2; Stanssens et al (1996) Proc. Natl. Acad. Sci. U.S.A., 93:2149) bind factor Xa prior to the formation of a quaternary inhibitory complex with the TF/factor VIIa complex. Small protein direct inhibitors (Dennis et al, 1994, J. Biol. Chem., 35:22137) and inactive forms of TF/factor VIIa are also known (Kirchhofer et al (1995) Arteriosclerosis, Thrombosis and Vascular Biol., 15:1098; Jang et al (1995) Circulation, 92:3041). Additionally, synthetic peptides and soluble forms of mutant TF which retain binding affinity but have reduced cofactor activity have been prepared (Roenning et al (1996) Thromb. Res., 82:73; Kelley et al, (1997) Blood, 89:3219). U.S. Pat. No. 5,679,639 describes polypeptides and antibodies which inhibit serine protease activity. U.S. Pat. No. 5,580,560 describes a mutant factor VIIa which has an improved half-life. U.S. Pat. No. 5,504,067 and U.S. Pat. No. 5,504,064 describe a truncated TF for the treatment of bleeding. Kunitz domain-tissue factor fusion proteins have also been shown to be bifunctional anticoagulants (Lee et al (1997) Biochemistry, 36:5607–5611). The TF/factor VIIa complex has been indicated as an attractive target for the development of inhibitors based on a dissociation between surgical bleeding and prevention of intravascular thrombosis (Harker et al (1995) Thromb. Haemostas., 74:464).
Compounds which block or inhibit enzymes in the coagulation cascade are therapeutically useful in treating or preventing thrombosis in a mammal suspected of having a condition characterized by abnormal thrombosis. For example, with respect to arterial vasculature, abnormal thrombus formation due to deterioration of an established atherosclerotic plaque is a major cause of acute myocardial infarction and unstable angina. Treatment of an occlusive coronary thrombus by thrombolytic therapy or percutaneous transluminal coronary angioplasty (PTCA) may be accompanied by reclosure of the vessel. In the venous vasculature, many patients undergoing surgery, particularly in the abdominal and lower body regions, experience thrombus formation which reduces blood flow and can lead to a pulmonary embolism. Disseminated intravascular coagulopathy in both the venous and arterial systems occurs commonly during septic shock, some viral infections, and cancer and may lead to rapid and widespread thrombus formation and organ failure.
Coumarin type, e.g. Warfarin, have certain therapeutic limitations, including excessive bleeding (minor and major hemorrhage. The typically slow onset of action (prothrombic) and long duration of action also complicate emergency procedures and necessitates frequent monitoring (Levine et al (1995) Chest 108 (4S), 276S; Lafata et al (2000) Thrombosis and Thrombolytics 9:S13; Marchetti et al (2001) Am. J. Med. 111:130; Garcia-Zozaya, I. (1998) J. of Kent. Med. Assoc. 96(4):143). Also, typically the cost of monitoring blood levels far exceeds the cost of coumarin and heparin type anticoagulant therapy.
PTCA and recanalization are favored procedures for treating occluded vessels. However, arterial thrombosis following these procedures remains a leading cause of failure. Heparin, the most widely used anticoagulant, has not been shown to be entirely effective in the treatment and prevention of acute arterial thrombosis or rethrombosis.
The synthesis and development of small molecule inhibitors based on the known three-dimensional structure of proteins is a challenge of modem drug development. Many thrombin inhibitors have been designed to have a hirudin-type structure. Stubbs and Bode, Current Opinion in Structural Biology 1994, 4:823–832. New synthetic thrombin inhibitors, as well as inhibitors of factor Xa and TF/factor VIIa, are reported. See, for example, Annual Reports in Medicinal Chemistry, 1995–1997, Academic Press, San Diego, Calif.; U.S. Pat. No. 5,589,173 and U.S. Pat. No. 5,399,487.
U.S. Pat. No. 6,472,393 and WO 00/41531 describe a class of inhibitors of serine proteases such as TF/factor VIIa, and which have acylsulfonamide and benzamidine moieties. These serine protease inhibitors have proven to have potent antithrombotic activity in vivo. However, there remains a need for potent TF/factor VIIa inhibitors that have optimized activity, selectivity and pharmacokinetic properties such as clearance, half life and bioavailability. Prodrug forms of TF/factor VIIa inhibitors may be employed to establish improved oral bioavailability.